This invention relates to a sealing material having a low sealing temperature and, in particular, to lead free glass used in the sealing material.
The sealing material has been used for sealing glass, ceramics and/or metals to each other. The sealing material is usually consists essentially of glass and fillers. For sealing electronic parts, the sealing material is desired to have a low sealing temperature.
Lead borate glass, typically, lead-zinc borate glass has conventionally been used as the low temperature glass in the sealing material which preferably has a sealing temperature of 430-500xc2x0 C. and a thermal expansion coefficient (TEC) of 70-100xc3x9710xe2x88x927/xc2x0 C.
However, it is desired to avoid use of lead compounds for the health and safety.
It is an object of this invention provide to lead-free tin silicate-phosphate glass of an improved mechanical strength and a weather resistance by using common oxide SiO2 as a glass forming element other than P2O5.
It is another object of this invention to provide a sealing material comprising fillers and lead-free tin silicate-phosphate glass containing silica as a glass forming element.
According to this invention, a lead-free tin silicate-phosphate glass as a sealing material is obtained which consists essentially of, by molecular percent, 30-80%, preferably, 40-60% SnO, 5.5-20%, preferably, 5.5-10% SiO2, 10-50%, preferably, 24.1-40% P2O5.
The lead free tin silicate-phosphate glass can contains at least one of glass stabilizing elements including 0-35% ZnO, 0-20% B2O3, 0-10% Al2O3, 0-20% WO3, 0-20% MoO3, 0-15% Nb2O5, 0-15% TiO2, 0-15% ZrO2, 0-35% R2O (R is Li, Na, K, and/or Cs), 0-10% CuO, 0-10% MnO, 0-15% Rxe2x80x2O (Rxe2x80x2 is Mg, Ca, Sr and/or Ba), a total content of at least one of the glass stabilizing elements being up to 40%.
According to an embodiment of this invention, a sealing material comprises the lead free tin silicate-phosphate glass described above of 50-100% in volume and the balance of refractory fillers. As refractory fillers, cordierite, tin dioxide, diniobium pentaoxide can be used. Zirconium phosphate, willemite and mullite can also be used as the fillers.
The lead-free glass used together with fillers in a sealing material according to this invention is a tin silicate-phosphate (SiO2xe2x80x94P2O5xe2x80x94SnO) glass which consists essentially of, by molecular percent, 30-80% SnO, 5.5-20% SiO2 and 10-50% P2O5.
SnO of 30-80% is contained to lower a melting point of the resultant glass. SnO content less than 30% excessively raises the viscosity of the resultant glass so that the sealing material using the glass has a disadvantageous high sealing temperature. If SnO content is more than 80%, the resultant material does not glass.
SiO2 is used as a glass-forming element and contained by an amount of 5.5-20%, preferably 5.5-10%. In use of SiO2 content less than 5.5%, the resultant glass has insufficient improvement of the mechanical strength and the weather resistance. SiO2 content more than 20% excessively raises the viscosity of the resultant glass.
P2O5 of 10-50%, preferably 24.1-40%, is contained together with SiO2 as glass-forming elements. If P2O5 is less than 10%, the resultant material does not form glass. If P2O5 is more than 50%, drawback inherent to phosphate glass is remarkably present in the resultant glass.
SiO2xe2x80x94P2O5xe2x80x94SnO glass according to this invention may contain at least one of glass stabilizing elements. The glass stabilizing elements include ZnO, B2O3, Al2O3, WO3, MoO3, Nb2O5, TiO2, ZrO2, R2O (R is Li, Na, K, and/or Cs), CuO, MnO Rxe2x80x2O (Rxe2x80x2 is Mg, Ca, Sr and/or Ba). The total content of at least one of the glass stabilizing elements is up to 40%. If the stabilizing-element content is more than 40%, the resultant glass is rather unstable and is readily devitrified at a shaping process, which is disadvantageous.
ZnO has a function for lowering a melting point as well as stabilizing glass and may be contained by 0-35%, preferably, 3-25%. ZnO content more than 35% increases crystallization of the resultant glass and lowers flowability of the glass.
B2O3 content is 0-20%, preferably, 0-10%, and more preferably 0-4.9%. B2O3 content more than 20% disadvantageously increases viscosity of the glass.
When B2O3 is contained in the glass, it is desired to determine (B2O3 content)/(P2O5 content) less than 0.20 in order to suppress raise of glass transition point.
Al2O3 content is 0-10%, preferably, 0-5%. Al2O3 content more than 10% disadvantageously increases viscosity of the glass.
Each of WO3 and MoO3 contents is 0-20%, preferably, 0-10%. When any one of them is contained more than 20%, the resultant glass increases in the viscosity disadvantageously.
Each of Nb2O5, TiO2, and ZrO2 contents is 0-15%, preferably, 0-10%. More than 15% content disadvantageously increases crystallization of the glass.
R2O content is 0-35%, preferably, 0-15%. More than 35% content disadvantageously increases crystallization of the glass.
Each of CuO and MnO contents is 0-10%, preferably, 0-5%. More than 10% makes the glass unstable.
Rxe2x80x2O content is 0-15%, preferably, 0-10%. More than 15% makes the glass unstable.
Further, it is possible to contain F so as to lower the melting point of the glass. The F content is selected to determine F/(F+O)xe2x89xa60.3, preferably, F/(F+O)xe2x89xa60.1, in molecular percent. When F/(F+O) is larger than 0.3, the glass is unstable.
The lead-free SiO2xe2x80x94P2O5xe2x80x94SnO glass according to this invention has a glass transition point of 250-350xc2x0 C. and an excellent flowability at a temperature of 500xc2x0 C. or less. The TEC of the glass is 90-150xc3x9710xe2x88x927/xc2x0 C.
The SiO2xe2x80x94P2O5xe2x80x94SnO glass according to this invention itself can be used as a sealing material, without fillers, for materials having a compatible TEC. In use for sealing other materials having TEC non-compatible to the glass, for example, alumina having TEC of 70xc3x9710xe2x88x927/xc2x0 C., window glass plate having TEC of 85xc3x9710xe2x88x927/xc2x0 C. and others, the glass of this invention is mixed with refractory fillers of low expansion coefficient and is used as a composite type sealing material. The refractory fillers can be mixed with the glass of this invention not only for adjusting the TEC but also for improving the mechanical strength of the seal as formed.
In a composite type sealing material, refractory fillers are mixed by 50 volume % or less with the glass of this invention of the balance. Mix of refractory fillers more than 50 volume % makes the flowability of the sealing material low to a level insufficient for sealing because the glass content is excessively low.
Though various refractory fillers can be used, cordierite, tin dioxide, and diniobium pentaoxide are preferable fillers because their mixture with the glass are stable in the alter glass. Zircon, zirconium phosphate, willemite, mullite, and the like can be used as fillers in the sealing material.
In production of the sealing material using the lead free SiO2xe2x80x94P2O5xe2x80x94SnO glass according to this invention, a glass batch is prepared and then melted to produce glass. The melting must be carried out with care so that SnO is not oxidized into SnO2 during the melting process. To this end, it is recommended that the melting is conducted in a non-oxidizing atmosphere such as N2 atmosphere.
Thereafter, the molten glass is shaped, ground and classified to obtain glass powder. Thereafter, refractory fillers are mixed with into the glass powder if it is desired. Thus, a powdery sealing material is obtained.
In use of the sealing material, the powdery sealing material is mixed with an organic solvent to form a paste. The paste is applied or coated onto bonding surfaces of target objects to be joined and sealed. Then, the target objects are brought into contact with each other at their bonding surfaces and are subjected to baking or firing at a condition where the bonding surfaces of the target objects get wet sufficiently. It is effective that the baking or firing is carried out in the non-oxidizing atmosphere.